Effect of Ru doping content on microstructural evolution and oxidation resistance of NiAlHf coatings at 1150 °C

• Ru addition promotes early θ- to α-Al 2 O 3 phase transformation during oxidation. • Cr(W,Ta)-rich bands and Cr 2 Hf/(Cr,Co) 2 Hf phases inhibit degradation and hinder Al diffusion. • Increasing Ru content enhances oxidation and spallation resistance of NiAlHf coatings. NiAlHf coatings were deposited on a nickel-based single-crystal René N5 substrate using arc ion plating technology. Isothermal oxidation was performed in laboratory air at 1150 °C for up to 200 h. The microstructural evolution and oxidation behavior were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, etc. The results indicate that Ru addition promotes the transformation from θ-Al 2 O 3 to α-Al 2 O 3 during oxide-scale formation, and the dense and continuous α-Al 2 O 3 scale effectively suppresses Al interdiffusion. Besides, the formation of Cr 2 Hf and (Cr,Co) 2 Hf precipitates within the coating, as well as a Cr(W,Ta)-rich precipitation band at the coating/substrate interface can act as effective diffusion barriers, which contributes to delayed microstructural degradation of the coating. After 200 h of oxidation, the average oxidation rates of the coating decrease from 0.1100 g/(m 2 ·h) to 0.0536 g/(m 2 ·h) with Ru content increases, accompanied by enhanced resistance to oxide-scale spallation. The mechanisms of Ru addition on phase transformation of θ-Al 2 O 3 to α-Al 2 O 3 and microstructural evolution of NiAlHf coating are also discussed.